1. Field of the Disclosure
This invention is related to power supplies. In particular, the invention is related to controllers for switching power supplies.
2. Background
In a typical application, an ac-dc power supply receives an input that is between 100 and 240 volts rms at a relatively low frequency that is nominally either 50 Hz or 60 Hz from an ordinary ac electrical outlet. The ac input voltage is usually rectified within the power supply to become a source of unregulated dc voltage for use by a dc-dc switching power converter. Switches in the power converter are typically switched on and off at a relatively high frequency (tens or hundreds of kilohertz) by a control circuit to provide a regulated output that may be suitable for operating an electronic device, or for charging a battery that provides power to an electronic device.
The design of a switching power supply is usually a compromise among conflicting requirements of efficiency, size, weight, and cost. The optimal solution that delivers the rated output power usually sets the switching frequency much higher than 20 kHz, outside the range of human hearing.
Regulatory requirements limit the amount of energy power supplies can consume when they operate at low loads, such as standby loads and at no load. When a switching power supply delivers much less than its rated power, the energy lost within the power supply is dominated by losses from the action of switching. Therefore, it is beneficial for the power supply to operate at lower switching frequencies when the output power is low to reduce the dominant losses.
The optimal switching frequency at low power often falls within the band of audio frequencies below 20 kHz. Switching within the band of audio frequencies can produce undesirable audio noise that results from mechanical excitation of electrical and magnetic components in the power supply. It is difficult to adjust the frequency of an oscillator in a continuous way to avoid the undesirable audio frequencies while meeting requirements for the power supply to be stable and to respond adequately to changes in load.
To overcome this difficulty, controllers for power supplies typically set an oscillator at a fixed frequency, and they regulate the output by allowing and preventing switching during the switching periods defined by the oscillator. The switching periods of the oscillator are sometimes referred to as switching cycles. The resulting groups of consecutive switching and non-switching periods produce an effective switching frequency that may be substantially less than the frequency of the oscillator. The effective switching frequency may be thought of as an average switching frequency that is substantially the fixed oscillator frequency multiplied by the ratio of the number of times switching occurs in a large number of consecutive switching periods divided by the large number of consecutive switching periods.
The switching periods where switching occurs are sometimes called enabled switching periods and the periods where switching is prevented are sometimes called disabled switching periods or skipped switching periods. It is important to distribute the enabled periods and the skipped periods in a way that avoids the generation of audio noise while allowing the power supply to switch often enough for it to respond adequately to changes in load. The requirement for galvanic isolation can place restrictions on the grouping of enabled periods and skipped periods.
Safety agencies generally require galvanic isolation between the input and the output of an ac to dc power supply. Galvanic isolation prevents dc current between the input and the output of the power supply. In other words, a high dc voltage applied between an input terminal and an output terminal of the power supply will produce no dc current between the input terminal and the output terminal of the power supply. The requirement for galvanic isolation is a complication that contributes to the cost of the power supply and to the difficulty of avoiding switching at undesirable audio frequencies.
A power supply with galvanic isolation must maintain an isolation barrier that electrically separates the input from the output such that circuits on the input side of the isolation barrier are galvanically isolated from the circuits on the output side of the isolation barrier. Energy must be transferred across the isolation barrier to provide power to the output, and information in the form of signals must be transferred across the isolation barrier to regulate the output. Galvanic isolation is typically achieved with electromagnetic and electro-optical devices. Electromagnetic devices such as transformers and coupled inductors are generally used to transfer energy between input and output to provide output power, whereas electro-optical devices are generally used to transfer signals between output and input to control the transfer of energy between input and output.
Efforts to reduce the cost of the power supply have focused on the elimination of electro-optical devices and their associated circuits. Alternative solutions generally use a single energy transfer element such as a transformer or coupled inductor to provide energy to the output and also to obtain the information necessary to control the output. The lowest cost configuration typically places the control circuit and a high voltage switch on the input side of the isolation barrier.
The controller obtains information about the output indirectly from observation of a voltage at either a winding of the energy transfer element or a winding of another switched electromagnetic element. The winding that provides the information is on the input side of the isolation barrier. To reduce cost and complexity further, the controller can also use the same winding of the energy transfer element to obtain information about the input to the power supply. A difficulty with the use of a switched magnetic element to obtain the information necessary to control the power supply is that the controller receives no information during periods where there is no switching. Therefore, the controller must force the switch to switch often enough for it to respond adequately to changes in the load.
The input side of the isolation barrier is sometimes referred to as the primary side, and the output side of the isolation barrier is sometimes referred to as the secondary side. Windings of the energy transfer element that are not galvanically isolated from the primary side are also primary side windings, sometimes called primary referenced windings. A winding on the primary side that is coupled to an input voltage and receives energy from the input voltage is sometimes referred to simply as the primary winding. Other primary referenced windings that deliver energy to circuits on the primary side may have names that describe their principal function, such as for example a bias winding, or for example a sense winding. Windings that are galvanically isolated from the primary side windings are secondary side windings, sometimes called output windings.
Power supply controllers that obtain information about an output on the secondary side from a winding on the primary side, especially when the information is in the form of a pulsating signal, are sometimes referred to as having primary side controllers, and the power supplies are said to operate with primary side control.
Existing controllers for power supplies that reduce the effective switching frequency by either allowing or preventing switching during groups of switching periods have difficulty meeting cost and performance requirements in galvanically isolated applications. A low-cost solution is needed for primary side controllers to avoid effective switching frequencies that fall within the range of undesirable audio frequencies while allowing adequate control of the output.